Investment casting, also referred to as the lost wax process, is a casting process which is particularly suited for the production of small metal parts having a high degree of dimensional accuracy. The investment casting process is widely used for the fabrication of blades and vanes for gas turbine engines. Articles produced by this process have the advantage of requiring only minimal processing following casting. This process is discussed in U.S. Pat. Nos. to Earl, 1,831,555; Watts, 3,590,905; Horton, 3,686,006, and Moren, 3,179,523 and 3,196,505.
Turbine efficiency is closely related to operating temperature. Demands for improved efficiency have resulted in the development of more heat resistant alloys. Surface condition also affects high temperature life and it is important that the finished casting have a good surface condition, one which is free from defects which may cause subsequent failures.
A technique which has been employed to improve the high temperature properties of superalloys is directional solidification. In this technique a molten casting slowly solidified at a controlled rate so that the interface between the molten and solidified portions of the alloy passes slowly along the longitudinal axis of the part. One result of this technique may be to produce a series of columnar grains with the longitudinal axis of the grains being oriented with the longitudinal axis of the casting. Improved longitudinal high temperature properties are obtained as a result of the reduction in grain boundary area perpendicular to the longitudinal axis. This technique is described in the VerSnyder U.S. Pat. No. 3,260,505 which is assigned to the present assignee.
In the past, a common problem with nickel base superalloys used at elevated temperatures was a lack of ductility at intermediate temperatures such as about 1400.degree. F. This lack of ductility was responsible for many failures of turbine parts. It was discovered that the addition of small amounts of hafnium to nickel base superalloys greatly improved the intermediate temperature of these alloys. Additionally such hafnium additions were found to improve the transverse mechanical properties of the castings. The addition of hafnium to superalloys is discussed in U.S. Pat. Nos. 3,677,747 and 3,711,337.
A casting defect has been observed in these hafnium containing alloys which has not previously been noted in superalloys. This defect is a surface defect having a resemblance to a crack or hot tear. The defect has been given the name chain porosity. This defect is found in castings having a change in cross sectional area of at least 1:2 in the vicinity where the change in cross section occurs. When the moving solidification interface passes from a portion of the casting having a small cross sectional area to a portion of the casting having a larger cross sectional area it is believed that a condition arises which leads to the formation to the surface defect in the partially solidified casting. Careful studies of the defect reveal that it has a crack-like morphology and that the root portion of the crack contains hafnium/hafnium oxide. Although chain porosity has only been observed in hafnium containing alloys, those skilled in the art will appreciate that the problem may arise in the future in alloys which do not contain hafnium as more advanced alloys are developed.
Accordingly it is the purpose of the present invention to disclose a mold which may be used for the directional solidification of nickel base superalloys which will substantially eliminate the problem known as chain porosity. A further object of the present invention is the description of the technique useful in producing composite molds useful for the elimination of chain porosity in nickel base superalloy castings.